The present invention relates generally to hydrocarbon conversion catalysts. Hydrocarbon conversion catalysts are materials that are used to achieve a higher efficiency with respect to the yields and/or the selectivity of chemical reactions conducted to enhance the value of hydrocarbons. An example of the use of such a catalyst is found in the Fluid Catalytic Cracking (FCC) process in petroleum refineries, In the FCC process, a hydrocarbon feedstock with a boiling point usually higher than 650° F. is reacted with a powdered catalyst to produce greater quantities of higher value products that include gasoline, light cycle oil, heavy cycle oil, liquified petroleum gas and other light gases. The catalyst enhances the product yields when compared to a similar reaction without the presence of the catalyst.
The present invention is focused on hydrocarbon conversion catalysts known in the Art as “fluid solid” catalysts and the materials used as components in such catalysts. These catalysts are designed such that even though the catalyst is a solid (solid particles) it behaves like a liquid when enough fluidizing media (e.g. vapors, air) are present. In general, the average particle size of a hydrocarbon conversion catalyst particle is between 60 and 90 microns. Two important parameters of such catalysts are the percent, by weight, of the total particles that have a particle size below 20 microns and the percent by weight of the total below 40 microns.
The first parameter (below 20 microns) is important as an indication of losses or potential losses, as hydrocarbon conversion systems are generally designed with a cutoff in efficiency at around 20 microns, and particles below 20 microns are more difficult to prevent from release to the atmosphere and therefore more likely to present an environmental problem. The losses can be detrimental both economically and environmentally. Cyclonic systems or cyclones and air separators are well known devices used in the Art to separate materials of different physical properties. In particular, cyclones can separate mixtures of solids with different particle size, density, relative mass or any combination thereof. Particles below 20 microns are lost very rapidly from the air cyclone process and either captured by higher efficiency systems downstream or emitted to the atmosphere. The second parameter, the 0-40 micron content, is also significant, as particles in the 20-40 micron range are important for fluidization in the hydrocarbon conversion system. Poor fluidization stemming from a relatively low component of smaller particles can result in reduced catalyst circulation or fluid bed instability.
The chemical nature of hydrocarbon conversion catalysts is well known to those versed in the Art. These catalysts generally contain zeolite Y in one or several forms (RE-Y, USY, RE-USY, CREY, etc) as the main source of activity and selectivity. Some other zeolites like ZSM-5 can be added to change the selectivities both as an intrinsic part of the catalyst or as a separate additive particle. Aluminas or silica aluminas of different properties are sometimes added to increase the activity of the catalyst for conversion of heavy molecules (bottoms upgrading). Clay is used as a filler that assists with the catalyst's chemical and physical properties. A binder, generally a low molecular weight oligomer of silica (Silica Sol), or aluminum chlohydrol (Alumina Sol) is commonly added to the mixture, which is spray dried to form the particles that are shipped for usage, with or without a post-treatment step to adjust the chemical composition and set the desired selectivities. Another type of commercially available hydrocarbon conversion catalyst, the so called “In-Situ” technology, takes preformed spray-dried particles of Kaolin clay and other materials such as silica and aluminas and chemically treats them to form Zeolite Y based hydrocarbon conversion catalysts.
Solid hydrocarbon conversion catalysts are commonly made by spray drying slurries that contain a mixture of the desired components that usually include, as discussed above, the zeolite Y, clay, alumina and a binder which can be based on silica sols, alumina sols or mixtures of both. In one particular technology, a highly peptizable alumina itself becomes the primary binder. In another technology, a particle containing clay treated at specific conditions is formed via spray drying and then is processed to grow zeolite Y. Other components such as silica or alumina, or both, can be added to the clay for specific properties.
Typically, the slurry is spray-dried to give a more or less spherical shaped particle. The particle size distribution of the spray-dried material is a function of the spray drier conditions and the nature and composition of the slurry. In general, the desired properties include: (1) a minimal amount of the 0-20 micron particles, (2) 10-20% of the particles being between 20-40 microns, and (3) an average particle size distribution between 65 microns and 85 microns. In general, hydrocarbon conversion catalysts are made as a continuum of particle sizes and compositions determined by the slurry properties and the spray drier conditions. In some cases, an air classifier is used to remove the smaller particles to meet certain specifications. In general, the 0-40 micron content can be controlled by the use of air classifiers, but this practice is costly, as the efficiency of the classifiers is poor, and valuable catalyst is commonly lost in this practice in order to meet a specification.
The chemical reactions that occur in the hydrocarbon conversion process can be diffusion limited. In general these reactions are defined as reactions in which mass transport of reactants into the catalyst particle limits the reaction efficiency. In commercial processes, many reactions occur, and some may be diffusion limited, while others are not. Another way to define such reactions is that the amount of reaction or the product yields are dependent on the particle size of the solid catalyst. These conditions of “diffusion limitation” are also common to many other catalytic processes. For example, nickel-containing hydrocarbons are very large, and react primarily on the outer-most layers of an FCC catalyst particle. This results in most of the nickel being deposited on the outside of the particle. An analysis of the total external area of the particles as a function of the radius of the particles shows that in such instances, the nickel will be preferentially deposited on the smaller particles due to a larger external area per unit of volume. Among the important hydrocarbon conversion processes that can be diffusion limited, and therefore in which mass transfer is key, are the solid-liquid/solid-solid reactions such as those for Biomass conversion of wood or cellulosic material in contact with a solid catalyst particle. In addition, other reactions in which the contact time is very short tend to also fall in the definition of diffusion limited reactions.
Just as nickel deposition on the hydrocarbon conversion catalyst particle is a function of particle size, many other reactions, some desirable, some undesirable, are dependent on the particle size of the hydrocarbon conversion catalyst.
In general, the catalyst composition of a hydrocarbon conversion catalyst is the same across particle sizes. When mixtures of additives are included, these additives may have a slightly different particle size distribution. However, it is a clear characteristic of hydrocarbon conversion catalysts and additives used to date, to have a continuous, smooth particle size distribution.
While interpreting the terms used to describe the present invention, it is important to consider the different techniques used to measure the particle size distribution of hydrocarbon conversion catalysts. When referring to particle size distribution, this discussion of the present invention refers to the actual physical size as measured by physical methods (like a screen) in which after minimizing agglomeration or attrition, fractions are measured by whether or not particles are able to pass through the screen without major external forces, with the exception of gravity and vibratory motion. In the case of light scattering methodologies, which are well-known to a person in the Art, the particle size distributions are continuous by mathematical manipulations of the experiments. Such continuum is an approximation and it is a limitation of the light scattering technique.
In general terms, the processes and resulting catalyst formulations of the present invention remove two major constraints, thereby enhancing the value of solid hydrocarbon conversion catalysts. Large particles can be detrimental to the processing of the largest molecules in a chemical reaction where a short contact time is required to optimize the reaction. Large molecules cannot effectively transfer to and from the inside of the particle, and the total external surface area of large particies is substantially less than that of a similar amount of catalyst with a smaller particle size. However, removal of large particles results in the concentration of smaller particles. This concentration of smaller particles can be beneficial if the initial content of particles below the minimum optimal threshold is below its optimum. However, a concentration of smaller particles can also be detrimental, if there are too many fine catalyst particles and, as a result, these particles are not properly retained by the hardware. Therefore, the catalyst composition may need to also have smaller particles removed for optimal performance. When the value of the catalyst is considerable, in order to enhance commercial viability, the removed particles may need to be reused in another, or the same, process. Maximum recovery can be achieved if the removed large particles can be reduced in size and if the removed smaller particles are converted into larger particles by re-spray-drying.
Such improvements on the control of particle size are not known in the Art and have not been attempted before. The removal of fine particles by air classification to minimize losses of fine particles is known in the Art. However, unlike the present invention, such practice, which helps to control physical losses from the process, results in catalyst with a much larger concentration of large particles that is often detrimental to the overall catalytic performance of the system. The use of the classified fines as fluidization aids is also known in the Art. However, the process of the present invention that involves the removal and reprocessing of the larger particles of the catalyst system is not known in the Art. On the contrary, although experts in the field acknowledge diffusion limitations, the leading work performed in the Art up to this time has been done under the assumption that the major limitation is at the molecular level, where the pore structure of the catalyst is the limitation. The present invention clearly shows, contrary to the commonly accepted viewpoint, that in many cases, the diffusion limitation is a mass transfer phenomena to and from the catalyst particle. This diffusion limitation is at a micron scale, rather than at the molecular scale commonly assumed in the Art. Molecular considerations imply diffusion limitations at a nanometer or angstrom scale. The present invention clearly demonstrates a previously unknown and unsuspected improvement on yield structure. The focus of the current Art has for decades been on improving a catalyst's pore structure, rather than the improvements on mass transfer phenomena at a much larger scale of the present invention.